Chicago ? Two new government studies indicate about 1 in 100 children have autism disorders — higher than a previous U.S. estimate of 1 in 150.

Greater awareness, broader definitions and spotting autism in younger children may explain some of the increase, federal health officials said.

“The concern here is that buried in these numbers is a true increase,” said Dr. Thomas Insel, director of the National Institute of Mental Health. “We’re going to have to think very hard about what we’re going to do for the 1 in 100.”

Figuring out how many children have autism is extremely difficult because diagnosis is based on a child’s behavior, said Dr. Susan E. Levy of the Children’s Hospital of Philadelphia and a member of the American Academy of Pediatrics subcommittee on autism.

The new estimate would mean about 673,000 American children have autism. Previous estimates put the number at about 560,000.

One of the studies stems from the 2007 National Survey of Children’s Health. The results were released today, and published in October’s Pediatrics.

In that study, based on telephone surveys, parents reported about 1 in 91 children, ages 3 to 17, had autism, including milder forms such as Asperger’s syndrome.